The present invention relates to a base carrier for a tracklaying vehicle, which has a crawler belt provided with bushings each mounted between a pair of opposed links; and a sprocket comprising a plurality of teeth engaging with the bushings respectively. The invention also relates to hard facing methods for sprocket teeth and bushings which constitute a base carrier.
Tracklaying vehicles such as bulldozers are usually operated on an unleveled ground and therefore their base carriers are subjected to extremely severe wear conditions through working because of the presence of sand and rocks. The wear rates of the crawler bushings and sprocket teeth are comparatively fast since they are subjected to considerably great contact pressure and used in such a situation that they are repeatedly brought into sliding contact with each other with sand and rocks entrapped therebetween. In the case of a bulldozer for use in mining oil sand for example, soil and sand are more likely to stick to its base carriers because of the high viscosity of oil sand. This aggravates the entrapment of sand and soil so that the base carriers for such a bulldozer wear at much faster rates than those of ordinary bulldozers. Therefore, there are strong demands for the development of base carrier parts improved in wear resistance.
In an attempt to impart wear resistance to parts which are locally required wear-resistance, a wear-resistant weld overlay having a composite structure composed of a matrix metal (untreated soft material) and hard particles or the like is applied to the surface of a base material. As one example of such a technique, a surface hardening method for tool joints is disclosed in Japanese Patent Publication (KOKOKU) Gazette No. 56-35986 (1981) according to which hard particles are dropped from a point proximate to the arc into a molten weld pool so as to precipitate on the bottom of the matrix and so as not to protrude from the surface of the a=matrix. In this surface hardening method, the effects of the sedimentation of the particles are: (1) since the particles do not protrude from the bead surfaces, a casing, that is, the mating member the joint, does not get damaged; and (2) the hardness of the metal (parent phase metal) which holds the precipitated cemented carbide particles increases, leading to improved wear resistance in the region where the cemented carbide particles have deposited. Another example is disclosed in U.S. Pat. No. 4,097,711. This patent proposes a surface treatment method for steel rollers comprising the steps of carburizing the surface of a material up to a depth equal to at least a half of the required depth of surface treatment; supplying particles for surface treatment to a molten weld pool and evenly dispersing them to obtain an effective hardness through the thickness of the whole molten part; applying thermal treatments to this molten part in order to obtain the required properties in the surface structure as well as in the base material. In this surface treatment method, it is admitted that the effect of the uniform dispersion of the particles is (1) an improvement in wear resistance and the effects of the carburization are (1) an improvement in the fatigue strength of the steel roller body and (2) an improvement in the wear resistance of the steel roller body.
However, these surface hardening methods reveal the following drawbacks when they are applied, as they are, to the base carrier parts of a tracklaying vehicle such as sprockets and bushing.
(1) During traveling, the sprocket and bushings of the base carrier intermittently come into contact with each other so that loads are repeatedly imposed on these parts. In the above conventional methods, cracks running transversely to the overlay beads are often created during formation of the weld overlay and stresses are repeatedly concentrated on the cracks, aggravating them. This causes exfoliation or partial chipping of the weld overlay and development of the cracks into the parts, resulting in breakage of the parts in themselves. Such cracks are created in the weld overlay not only during formation of the weld overlay, but also during travel of the tracklaying vehicle due to repetitive loads in cases where the weld overlay is inhomogeneous or the shape of the overlay surface is uneven. In the latter case, there would also occur exfoliation and chipping of the weld overlay, and, still more, breakage of the parts per se. However, the above conventional methods do not teach any techniques for preventing cracking which occurs during formation of the weld overlay; cracking which occurs during travel due to the inhomogeneous structure of the weld overlay or unevenness in the surface of the weld overlay; and exfoliation, chipping and extension of cracks.
(2) Cracking due to repetitive loads during travel occurs not only in the weld overlay but also on the surface of the bushing body. Such cracking develops similarly to the above-mentioned cracking, sometimes leading to breakage of the bushing. U.S. Pat. No. 4,097,711 teaches a carburizing process capable of improving fatigue strength, according to which a weld overlay is formed after carburization and then quenching/tempering is carried out. In other words, a weld overlay is applied to high-carbonized steel. This process, however, presents the problem that the area near the boundary (the toe of the weld overlay) between the weld overlay and the body of the par is hardened and liable to cracking during weld overlay formation.
(3) Since the hard particles dispersed in the weld overlay have much higher wear resistance than the matrix (parent phase metal) which holds the hard particles, wear progresses with the hard particles coming to project from the weld overlay and the worn surface with the protrudent hard particles scrapes off a mating member to be engaged with the part, with the result that wear is still more expedited. The above conventional method does not discuss the problem attributable to wear patterns and its solution as well as the relationship between wear patterns and the preferable structure of the weld overlay.
The present invention is directed to overcoming the foregoing problems and a first object of the invention is therefore to provide a base carrier for a tracklaying vehicle, the base carrier being designed such that prompt conformability with a mating member is ensured by utilizing wear which occurs at the initial stage of traveling; exfoliation nor chipping is not caused in the weld overlay in engagement of the part with its mating member; cracking which has occurred during overlaying is prevented from extending, leading to breakage of the part; no new cracking occurs nor develops when the part is in operation; and the mating member is not adversely affected by the part during travel.
A second object of the invention is to provide a hard facing method for sprocket teeth which method meets the first object and has the capability of further improving wear resistance by providing a weld overlay with a property of prompt conformability relative to a mating member by utilizing wear at the initial stage of traveling; preventing occurrence of cracking as much as possible during overlaying in order to prevent exfoliation and chipping of the weld overlay during operation; stabilizing the quality of the weld overlay, that is, the amount and distribution of hard particles and the structure and hardness of a parent phase metal; and rendering the wear surface smooth during service so that damage to the mating member and falling and chipping of the part, which are attributable to scraping of the mating member due to the projection of the hard particles, are all prevented.
A third object of the invention is to provide a hard facing method for bushings, which method meets the first object and has the capability of further improving wear resistance by providing a weld overlay with a property of prompt conformability relative to a mating member by utilizing wear at the initial stage of traveling; preventing occurrence of cracking as much as possible during overlaying in order to prevent exfoliation and chipping of the weld overlay during operation; stabilizing the quality of the weld overlay, that is, the amount and distribution of hard particles and the structure and hardness of a parent phase metal; increasing the hardness of the metal (parent phase metal) which holds the hard particles of the weld overlay, by thermal treatments without causing quenching cracks; increasing the wear resistance and fatigue strength of the bushing body; and rendering the wear surface smooth during service so that damage to the mating member and falling and chipping of the part, which are attributable to scraping of the mating member by the projection of the hard particles, are all prevented.
The first object can be accomplished by a base carrier for a tracklaying vehicle according to a first aspect of the invention (associated with Claim 1), the base carrier comprising a crawler belt having bushings each mounted between a pair of opposed links and a sprocket having a plurality of teeth meshing with the bushings respectively,
each of the sprocket teeth having a welt overlay containing hard particles and composed of beads each running in a direction transverse to the rotating direction of the sprocket.
According to the invention, the overlaying direction of the sprocket teeth is a direction (which is generally equal to the direction of the tooth traces) transverse to the rotating direction of the sprocket, and therefore the direction (which is perpendicular to the tooth traces) of tensile stress imposed on the weld overlay when the sprocket comes in engagement with its mating member (i.e., bushings) is generally coincident with the direction (which is generally perpendicular to the beads) of cracking in the weld overlay. As a result, cracks can be prevented from opening, in other words, expanding. The weld overlay is preferably formed by forming narrow overlay beads so as to align successively in parallel. This stabilizes the quality of the overlay beads, namely, the amount and distribution of the hard particles and the structure and hardness of the matrix, which leads to the stable quality of the weld overlay. The overlaying direction transverse to the rotating direction of the sprocket contributes to an improvement in the initial conformity of the sprocket relative to the bushings when the sprocket comes in engagement with the bushings. When forming the overlay beads successively in parallel, the overlapped areas of the beads respectively assume a concave form. Excavated soil is entrapped and accumulated in this concave area, serving as an abrasive which makes a wear surface smooth at the initial stage of wear. If the unevenness of the weld overlay does not disappear at the initial wear stage, stresses will be concentrated on the concave areas so that cracks due to fatigue will be created, causing the problems mentioned earlier. It is understood from the foregoing description that these effects reach their culmination when the direction of the overlay beads is perpendicular to the rotating direction of the sprocket.
In the invention, the hard particles are preferably distributed in a less amount at the tip of each tooth than at the dedendum thereof and/or not contained at the end of the tip (this feature is associated with Claim 2). When the sprocket is in engagement with the bushings, the teeth of the sprocket elastically deform to a greater extent at the tip than at the dedendum. If the amount of deformation exceeds the allowable amount of deformation for the weld overlay, cracking will occur and develop in the weld overlay, leading to exfoliation. The more the hard particles are contained, the less the allowable deformation amount becomes, or the thicker the weld overlay containing the hard particles distributed therein are, the less the allowable deformation amount becomes. Accordingly, the hard particles are contained in a less amount in the area close to the tooth tip and are not contained at the tooth top, whereby exfoliation and chipping of the weld overlay can be prevented and, in consequence, the durability of the weld overlay can be stabilized.
Preferably, the hard particles are distributed in a more amount in the area intermediate between the dedendum and the tooth tip than at the dedendum and at the tooth tip (this feature is associated with Claim 3). With this arrangement, tenacity can be mainly imparted to the dedendum and the tooth tip while wear resistance can be mainly imparted to the area intermediate between the dedendum and the tooth tip, which leads to an improvement in the durability of the weld overlay.
It is preferable that the hard particles be contained in the weld overlay so as not to protrude from the surface of the weld overlay and so as to be densely distributed in the bottom area of the weld overlay (this feature is associated with Claim 4). Unless the hard particles protrude from the surface of the weld overlay, no damage will be given to the mating member to be engaged which would otherwise be scratched by the projecting hard particles. By densely distributing the hard particles at the bottom area of the weld overlay, in other words, by distributing the hard particles in a slight amount in the area near the surface of the weld overlay, the area near the surface of the weld overlay wears fast at the initial stage of wear so that the unevenness of the weld overlay can be removed to provide a smooth surface for engagement. This arrangement has the effect of eliminating, at an early stage, a source of stress concentration due to the unevenness of the surface of the weld overlay.
In the invention, the spacing between adjacent ones of the hard particles densely distributed in the bottom area of the weld overlay is preferably less than or equal to the size of the particles of soil to be entrapped (this feature is associated with Claim 5). The hard particles have higher wear resistance than the matrix (parent phase metal) which holds the hard particles. If the size of soil particles is smaller than the spacing between the hard particles, the matrix will wear in preference to the hard particles, so that the wear will progress with the hard particles projecting from the surface of the weld overlay. Such projecting hard particles will scrape the mating member off, expediting wear. By making the spacing between the hard particles less than or equal to the size of soil particles, the preferential wear of the matrix can be reduced, thereby lessening the projecting amount of the hard particles.
According to a second aspect (associated with Claim 6) of the invention, there is provided a base carrier for a tracklaying vehicle, the base carrier comprising a crawler belt having bushings each mounted between a pair of opposed links and a sprocket having a plurality of teeth meshing with the bushings respectively,
wherein a weld overlay containing hard particles is formed on the outer peripheral surface of each of the bushings, the outer peripheral surface serving as a contact surface for the sprocket that engages with the bushings and wherein the core of each bushing is made of steel of high tenacity.
In the invention, the outer peripheral surface of each bushing is preferably provided with a hardened layer (this feature is associated with Claim 7).
A bushing used for the base carrier of a tracklaying vehicle is required to have wear resistance at its outer periphery as well as fatigue strength in order to withstand repetitive loads imposed on the bushing. According to the invention, wear resistance is imparted to the most wear-vulnerable area (i.e., the contact surface for the sprocket) of the bushing by forming a weld overlay containing hard particles in the area; the wear resistance and fatigue strength of other peripheral areas than the contact surface for the sprocket are improved by forming a hardened layer on the surface of the bushing; and the core of the bushing is formed from highly tenacious steel in order to improve the tenacity of the bushing itself and to prevent crack extension. As a guide, the hardness of the hardened layer on the bushing surface is preferably HRC 45 or more. The provision of the hardened layer on the bushing surface has the effect of preventing occurrence of cracking in the vicinity of the end (i.e., the area near the boundary between the weld overlay and the busing body) of the weld overlay.
The provision of the hardened layer has the effect of preventing the preferential wear of the area close to the end of the weld overlay. If there is provided no hardened layer, the area close to the end of the weld overlay will wear preferentially, being scooped away, which leads to a decrease in the strength of the bushing. As a result, the bushing sometimes is broken before the weld overlay fully exerts its wear resistance. Preferably, the core of the bushing has a quench tempered structure having a hardness of HRC 26 to 45 (this is a measure). If the hardness of the bushing core is lower than it, the bushing per se will be deformed into a flat shape. On the other hand, if the hardness is higher than it, cracks which have occurred in the weld overlay or in the area close to the end of the weld overlay will quickly extend to the base material, resulting in breakage.
In the invention, the weld overlay is preferably formed such that each bead extends in a direction transverse to the rotating direction of the sprocket (this feature is associated with Claim 8). By making the overlaying direction of the outer peripheral surface of the bushing transverse to the rotating direction of the sprocket, the direction of tensile stress imposed on the weld overlay (i.e., the circumferential direction of the bushing) when the bushing comes into engagement with the mating member, sprocket can be substantially coincident with the direction of cracking on the weld overlay (i.e., the direction perpendicular to the beads), so that the cracks can be prevented from opening, in other words, expanding. It is also preferable to form the weld overlay by arranging narrow overlay beads successively in parallel. With this arrangement, the quality of the overlay beads, that is, the amount and distribution of the hard particles and the structure and hardness of the matrix can be stabilized, so that the quality of the weld overlay can be stabilized. In addition, the direction of overlaying, which is transverse to the rotating direction of the sprocket, has the effect of improving initial conformity at the time of engagement of the bushing with the sprocket. As mentioned earlier, where the overlay beads are formed so as to alien successively in parallel, the overlapped areas of the adjacent beads are concave in form. Therefore, soil and sand are entrapped and accumulated in these areas and serve as an abrasive, making the wear surface smooth at the initial stage of wear. If the unevenness of the weld overlay is not eliminated at the initial wear stage, stresses will be concentrated on the concave areas, causing fatigue cracking which raises the above-described problems. As apparent from the foregoing description, these operational effects reach their culmination when each overlay bead is arranged so as to extend in a direction perpendicular to the rotating direction of the sprocket.
In the invention, the weld overlay is preferably so formed on each bushing as to extend circumferentially over a substantially half of the outer peripheral surface of the bushing, the outer peripheral surface engaging with the sprocket (this feature is associated with Claim 9). With this arrangement, wear resistance can be imparted only to the contact surface for the mating member, sprocket in engagement with the bushing. This reduces manufacturing cost, the amount of deformation that occurs during formation of the weld overlay and during thermal treatment, and thermal stress as well as transformation stress. In addition, occurrence of cracking can be prevented and the necessity for bore machining after formation of the weld overlay can be eliminated.
The hard particles are preferably contained in the weld overlay in such a manner that they do not come out from the surface of the weld overlay and are densely distributed in the bottom area of the weld overlay (this feature is associated with Claim 10). Unless the hard particles come out from the surface of the weld overlay, damage to the mating member due to scratching by these particles can be avoided. By densely distributing the hard particles in the bottom area of the weld overlay, in other words, by scarcely distributing the hard particles in the near surface area of the weld overlay, the near surface area of the weld overlay is allowed to be quickly worn away at the initial stage of wear, so that the unevenness of the surface of the weld overlap is removed, forming a smooth engagement surface. Accordingly, this has the effect of eliminating, at an early stage, a stress concentration source presented by the unevenness of the overlay surface, as mentioned earlier.
In the invention, the hard particles are preferably contained in the weld overlay so as to be distributed at spacings lees than or equal to the size of soil and sand to be entrapped (this feature corresponds to Claim 11). The hard particles has higher wear resistance than the matrix (parent phase metal) which holds the hard particles. If the size of soil particles is smaller than the spacing between the hard particles, the matrix will wear in preference to the hard particles, so that wear will progress with the hard particles projecting from the overlay surface. In consequence, the projecting hard particles scrape the mating member off, expediting wear. By making the spacing between the hard particles less than or equal to the size of soil particles, the preferential wear of the matrix can be reduced, thereby lessening the projecting amount of the hard particles.
In addition, the weld overlay is so formed that strip-like overlay beads containing the hard particles are aligned successively in parallel (this feature corresponds to Claim 12), or alternatively strip-like overlay beads containing the hard particles and strip-like overlay beads containing no hard particles are alternately successively aligned in parallel (this feature corresponds to Claim 13). In the case of the latter overlaying pattern, the weld overlay is also formed such that each bead extends in a direction transverse to the rotating direction of the sprocket teeth. Therefore, each strip-like overlay bead containing no hard particles is protected by the two strip-like overlay beads containing the hard particles which are located on both sides of the former bead, so that this overlay pattern functions satisfactorily. The latter pattern has the advantage that a less amount of hard particles is used, leading to a saving of cost.
According to a third aspect (associated with Claim 14) of the invention, there is provided a base carrier for a tracklaying vehicle, the base carrier comprising, in combination, the sprocket associated with the first aspect and the bushings associated with the second aspect.
According to this feature, the synergistic erect of the characteristics of the weld overlay applied to the sprocket and the weld overlay applied to each bushing provides a base carrier having more improved wear resistance and well suited for use in a bulldozer used for, for instance, mining of oil sand. The wear resistance characteristic of the weld overlay applied to the sprocket is equivalent to that of the weld overlay applied to each bushing and therefore, an undesirable event, that is, scraping of the mating member by the hard particles can be avoided and wear rate can be minimized more effectively than the case in which either the sprocket or the bushings are provided with a weld overlay.
The second object can be accomplished by a hard facing method for sprocket teeth according to a fourth aspect (associated with Claim 15) of the invention. This method is for applying a weld overlay to each of the tooth flanks of a sprocket having a plurality of teeth to produce a wear-resistant sprocket teeth,
wherein overlaying is successively carried out such that each bead extends in a direction transverse to the rotating direction of the sprocket teeth and interpass temperature is controlled so as to make the amount of heat input for each tooth substantially constant during the overlaying.
According to the invention, the occurrence of cracking during weld overlay formation can be reduced. As to cracks which have already occurred, they can be prevented from opening, namely, extending, thanks to the arrangement in which the overlaying direction of the teeth of the sprocket is a direction (which is generally equal to the direction of the tooth traces) transverse to the rotating direction of the sprocket teeth, so that the direction (which is perpendicular to the beads) of cracking occurring in the weld overlay is generally coincident with the direction (which is perpendicular to the tooth traces) of stresses imposed on the weld overlay in service. In addition, the above arrangement makes it possible to improve the initial conformability of the sprocket teeth with respect to their mating members, the bushings and to let the weld overlay be smoothly worn. Further, overlaying passes are successively performed in a direction from each tooth tip to each dedendum ad interpass temperature is so controlled that the amount of heat input for every tooth during overlaying becomes substantially constant, so that the appearance of the beads can be smoothed and the occurrence of cracking due to the stress concentration on the concave portions formed on the surfaces of the beads can be prevented to stabilize the beads and, in consequence, the quality of the weld overlay. In this way, a further improvement in the wear resistance of the sprocket teeth can be achieved.
Preferably, overlaying is carried out in such a way that hard particles are added to a molten weld pool being formed on a base material of the sprocket teeth by means of an arc to form, on the base material, a weld overlay containing the hard particles (this feature is associated with Claim 16). This arrangement contributes to an improvement in the hardness of the molten metal, leading to improve wear resistance.
The combination of the supply of the hard particles and the control of the amount of heat input enables stabilization of the amount of the hard particles contained in the weld overlay, which leads to a further improvement in the quality of the weld overlay.
It is preferable to perform the overlaying of the invention with some latitude allowed in the grain size distribution of the hard particles for allowing dense charge with the hard particles (this feature is associated with Claim 17). According to this feature, the minimum grain size, which permits the presence of a large number of hard particles in a non-molten condition, is selected and some latitude is allowed in the grain size distribution. With this arrangement, small particles can get into the gaps between large particles which enables dense charge with the particles. As a result, the distance between the particles becomes extremely small, which reduces the unevenness of the wear surface caused by the projection of the particles in the case of steady wear.
An alternative preferred method for solving the problem of damage to a part itself or its mating member to smooth the wear surface as much as possible by preventing the projection of the hard particles is preferably designed such that cemented carbide is used as he hard particles and the components of the cemented carbide are allowed to melt into molten metal so that carbide precipitates over the entire area of a matrix surrounding the cemented carbide (this feature corresponds to Claim 18). Increases in the amounts of weld heat input increase the floating amount of the constituents of the hard particles, causing precipitation of double carbide, Fexe2x80x94Wxe2x80x94C within the matrix. Therefore, the hardness of the matrix increases, while the hardness of the cemented carbide decreases because of deterioration. The difference in hardness between the matrix and the hard particles is, accordingly, reduced. In such a weld overlay, the matrix and the hard particles wear at the substantially same wear rate so that a smooth wear surface with no hard particles protruding therefrom can be attained. Thus, the foregoing problem can be solved.
The overlaying of the invention is preferably carried out while the amount of hard particles to be supplied being controlled such that the hard particles are not contained in the tooth tip (this feature corresponds to Claim 19). This arrangement prevents, without fail, exfoliation and chipping of the weld overlay at the tooth tip which would occur due to a collision between the base material at the tooth tip and its mating member and due to elastic deformation.
The overlaying of the invention is preferably carried out while overlaying rate or the amount of hard particles to be supplied being controlled such that the weld overlay formed near the tooth tip becomes thin or alternatively such that the hard particles contained in the weld overlay near the tooth tip become scarce (this feature corresponds to Claim 20). With this arrangement, exfoliation of the weld overlay at the tooth tip can be prevented.
In the invention, it is preferable to apply quench/tempering and preheating to the sprocket teeth prior to overlaying (this feature corresponds to Claim 21). It is also preferable to carry out postheating after overlaying, for alleviating surface residual stresses imposed on the sprocket teeth (this feature corresponds to Claim 22). According to this feature, the hardness of the base material can be increased. Additionally, tensile residual stresses generated during the formation of the weld overlay can be alleviated by carrying out preheating and postheating. In consequence, cracking that would occur throughout the entire weld overlay because of the residual stresses can be avoided.
In the invention, after overlaying, weld deformation which occurred in the sprocket teeth may be corrected and press quenching may be carried out to remove portions affected by weld heat (this feature corresponds to Claim 23). With this arrangement, it is possible to easily correct, by press quenching, deformation which occurred during overlaying, so that the sprocket teeth can be mounted by correctly abutting its surface on the mating member, in other words, deformation attributable to attachment looseness and occurring during operation can be prevented. In addition, a brittle, heat-affected structure including coarsened crystal grains and formed in overlaying can be eliminated. As a result, the fatigue strength of the sprocket teeth can be improved.
The third object of the invention can be accomplished by a hard facing method for bushings according to a fifth aspect (associated with Claim 24) of the invention. This method is for applying a weld overlay to the outer peripheral surface of a bushing to produce a wear-resistant bushing, the inner and outer peripheral surfaces of the bushing having been subjected to machining, the method comprising:
(a) a weld overlay formation step for supplying hard particles to a molten weld pool being formed on a base material of the bushing by use of an arc to form a weld overlay containing the hard particles on the base material;
(b) a first thermal treatment step for applying gas carburization to the bushing on which the weld overlay has been formed; and
(c) a second thermal treatment step for applying reheating/quenching and tempering to the busing after the first thermal treatment step.
According to the hard facing of the invention for a bushing, in the weld overlay formation step, a weld overlay containing hard particles is firstly formed on the base material of a bushing by forming a molten weld pool on the base material of the bushing by use of an arc while supplying the hard particles to the molten weld pool. Then, in the first thermal treatment step, the bushing after the weld overlay formation is subjected to gas carburization. In the second thermal treatment step, the bushing is reheated, quenched and then tempered. Instead of these steps, a consecutive thermal treatment step may be employed in which quenching is performed immediately after gas carburization without reheating. By applying the thermal treatment to the bushing after the formation of the weld overlay, fine spherical carbide granules are uniformly dispersed on the upper part of the matrix of the weld overlay because of carbon which has penetrated from outside during carburization. In addition, fine martensite is dispersed by quenching thereby obtaining increased hardness and therefore a structure of increased wear resistance. On the lower part of the matrix, wear resistance is enhanced by the hard particles and carbon floating from the hard particles develops into carbide so that a structure of increased wear resistance can be obtained. In the case of xe2x80x9cintermetallic wear partsxe2x80x9d such as bushings which have a metal-to-metal contact relationship with their mating parts, it is preferred that the size of the hard particles be smaller than that of soil and sand. The reason for this is that if the size of the hard particles is larger than that of soil and sand, the matrix between the hard particles is preferentially scraped off by soil and sand so that the hard particles project from the wear space, resulting in falling off of the hard particles or scraping off of the mating parts. To improve the wear resistance of the matrix of the weld overlay is also important particularly for xe2x80x9cintermetallic wear partsxe2x80x9d. If the wear resistance of the matrix is considerably lower than that of the hard particles, the matrix will preferentially wear and the hard particles will project from the wear surface. Hence, in the invention, the matrix of the weld overlay is reinforced and the sufficient hardness of the base material is ensured by adapting the thermal treatment subsequent to the formation of the weld overlay whereby a bushing excellent in wear resistance as well as in fatigue strength can be attained.
According to a sixth aspect (associated with Claim 25) of the invention, there is provided a hard facing method for bushings, which method is for applying a weld overlay to the outer peripheral surface of a bushing to produce a wear-resistant bushing, the inner and outer peripheral surfaces of the bushing having been subjected to machining, the method comprising:
(a) a weld overlay formation step for supplying hard particles to a molten weld pool being formed on a base material of the bushing by use of an arc to form a weld overlay containing the hard particles on the base material;
(b) a first thermal treatment step for applying non-oxidative heating/oil hardening and tempering to the bushing on which the weld overlay has been formed; and
(c) a second thermal treatment step for carrying out, subsequently to the first thermal treatment step, inner-peripheral-surface hardening and tempering, the inner-peripheral-surface hardening being carried out in such a manner that the inner peripheral surface of the bushing is subjected to induction heating while the outer peripheral surface thereof being cooled.
According to the hard facing of the invention for bushings, in the weld overlay formation step, a weld overlay containing hard particles is firstly formed on the base material of a bushing by forming a molten weld pool on the base material of the bushing by use of an arc while supplying the hard particles to the molten weld pool. Subsequently, in the first thermal treatment step, the bushing after the weld overlay formation is subjected to non-oxidative heating, oil hardening and then tempering. In the second thermal treatment step, inner-peripheral-surface hardening is performed in such a way that the inner peripheral surface of the bushing is induction-heated, while its outer peripheral surface being cooled, and finally, tempering is performed. With this hard facing method, a bushing having improved wear resistance and fatigue strength which are equivalent to those of the bushing of the first aspect can be achieved.
In the fifth and sixth aspects of the invention, the weld overlay formation is carried out with some latitude in the grain size distribution of the hard particles for allowing dense charge with the hard particles (this feature corresponds to Claim 26). Soil and sand entrapped between the parts are ground in an order of several xcexcm, whereas it is impossible in view of the process to shorten the spacing between the unmolten, hard particles to the extent equivalent to the above size because the hard particles will be mostly melted in doing so. Therefore, the invention is arranged such that the minimal grain size, which allows the presence of a large number of hard particles in an unmolten state, is selected and some latitude is allowed in the grain size distribution in order to let small particles penetrate into the gaps between large particles. With this arrangement, the base material can be more densely charged with the hard particles, thereby making the spacing between the particles extremely small. As a result, the unevenness of the wear surface due to the projection of the particles in steady wear can be reduced.
As an alternative method for solving the problem of chipping of a part and damage to its mating member to obtain a smooth wear surface by preventing the projection of the hard particles, it is preferable for the fifth and sixth aspects of the invention to use cemented carbide as the hard particles and to allow the components of the cemented carbide to melt into molten metal thereby precipitating carbide over the entire area of the matrix surrounding the cemented carbide (this feature corresponds to Claim 27). As the amount of weld heat input increases, the amount of components of the hard particles to be melted out increases so that double carbide, Fexe2x80x94Wxe2x80x94C precipitates in the matrix increasing the hardness of the matrix, while the cemented carbide decreases in hardness because of deterioration. As a result, the difference in hardness between the matrix and the hard particles is reduced. In such a weld overlay, the matrix and the hard particles wear at the substantially same rate and as a result, the hard particles do not project, a smooth wear surface is attained, and therefore the foregoing problem can be solved.
For imparting a desired hardness to the base material, it is preferable to use a substance having good hardenability. On one hand, good hardenability is advantageous, but, (in the other hand, good hardenability is likely to cause cold cracking after formation of a weld overlay. To prevent cold cracking, it is preferable for each aspect of the invention to carry out preheating in which the bushing is heated to a predetermined temperature (e.g., 250xc2x0 C. or more before forming the weld overlay (this feature corresponds to Claim 28).
In each aspect of the invention, there is no cracking problem in the case where the first thermal treatment process starts immediately after the weld overlay formation step. However, there is a high probability of cracking due to tensile residual stress generating in the weld overlay formation step in the case where the bushing is once cooled after the weld overlay formation step and particularly in the case where a preheating treatment is not applied before the weld overlay formation step. This residual stress increases with decreases in temperature from 500xc2x0 C. or less at which plastic deformation is unlikely to occur. Therefore, if no preheating treatment process is adapted, the difference in yield between the weld overlay and the base material is significant and therefore residual stress increases sharply, so that residual stress exceeds its breaking stress point, accompanied with cracking at an early stage after the formation of the weld overlay. This cracking occurs, with a large opening, transversely to the beads in a direction perpendicular to the direction of welding. Where a preheating treatment process is adapted, residual stress increases slowly and reaches its breaking stress point after temperature drops. Therefore, cracking accompanied with a large opening scarcely occurs but a number of minute cracks are created over the entire weld overlay. For preventing this, it is preferable to carry out post heating in which the bushing is heated to a predetermined temperature (e.g., 350xc2x0 C.) after the weld overlay formation step (this feature corresponds to Claim 29). The provision of the post heating treatment process after the weld overlay formation step alleviates residual stress and prevents occurrence of minute cracks.
In each aspect of the invention, press-fit-portion grinding, end face grinding, and end face burnishing may be applied to the outer peripheral surface of the bushing as finishing after the second thermal treatment step (this feature corresponds to Claim 30). With this arrangement, the desired finished product which has undergone surface finishing can be obtained.